The recovery of acids from aqueous solutions by means of membrane processes is well known. The many membrane processes include dialysis and electrodialysis.
Dialysis is generally carried out in a multi-compartment unit wherein dialyzate compartments are alternating with diffusate compartments separated from each other by suitable membranes. When acidic solutions are treated for the recovery of a diffusate stream containing a major portion of the acid, anionic membranes are used that selectively allow the acid anion as well as protons to pass from the dialyzate side to the diffusate side of the membranes. The dialysis method for treating acidic solutions has the main disadvantage of yielding an acid product of a relatively low concentration because the driving force is mainly determined by the acid concentration gradient across the membranes. To obtain an efficient production rate and a high acid concentration in the product stream, large equipment with a high membrane area is required.
Electrodialysis is generally carried out in a unit that comprises a multiplicity of alternating anionic and cationic permselective membranes defining alternating diluate and concentrate compartments between an anode compartment containing an anode and a cathode compartment containing a cathode. The electrodes are connected to a source of direct electrical current. Electrodialysis is more effective than dialysis for concentrating dilute acid solutions, but has the disadvantage that, when the acid solutions contain cations, the concentrated acid solution often also contains undesirable concentrations of those cations. Another disadvantage of electrodialysis is low current efficiency due to proton leakage through the anion exchange membranes. In order to improve the efficiency of acid transfer, it is necessary to select membranes carefully, as well as to operate at moderately high current densities. Such high current densities, however, result in a decreased selectivity of the acid removal.
The prior art contains many references on the use of either dialysis or electrodialysis for the separation or concentration of acids in solutions. In Jap. Kokai 53-19171 (1978) there is described a method for separating and recovering both acids and metals from acid and metal-containing solutions. Solution is fed to a dialysis tank for the removal of acid, and resulting solution is then subjected to membrane electrolysis for the electrodeposition of metals. The metal-reduced solution is subjected to electrodialysis from which the concentrate, i.e. solution concentrated in acids and metals, is fed to the membrane electrolysis tank and the dilute solution is returned to the dialysis step. This appears to be one of the few processes in which both dialysis and electrodialysis are used.